Over the last number of years animal and clinical experiments have shown the practicality of providing synthetic or artificial tooth roots which can be anchored in the jawbone of a patient and which can serve as supports for dental prosthetics. Such implants have been found to restore the bite and chewing quality in general and to improve the peridontal tissue in the remaining teeth of the patient by contrast with removable prostheses or plates which encounter all types of problems including tissue deterioration, poor fit and irritation, etc.
The loss of permanent side teeth tends to produce gaps which are detrimental to the patient because of the tendency of the remaining teeth to move in the direction of such gaps.
It has been proposed to fill such gaps with bridges attached to remaining teeth, but this procedure has problems as well, not only because of the undue stress which is applied to the support teeth but because the yieldability of the mesial support teeth results in a high degree of mobility of the bridge structure.
Disadvantages of such systems can be avoided at least in part by synthetic root implants which do not possess this kind of mobility and which are able to withstand the stresses which may be applied to a bridge supported on such implants. Nevertheless a certain degree of mobility or cushioning effect is desired for a number of resons. For example, the chewing characteristics are not as desirable if a prosthetic tooth is less moble than adjacent or proximal remaining teeth. The dental prosthesis should have the same mobility, within limits, as normal teeth if peridontal tissue damage is to be avoided.
Furthermore, some degree of cushioning is required to prevent overloading or overstressing of the prosthesis.
Consequently, it is desirable to have a certain degree of cushioning between the prosthetic tooth or the dental prosthesis and the implant or synthetic root. If an elastic cushion is provided to allow deformation and storage of the potential energy resulting from the kinetic energy of chewing, some of the problems outlined above can be solved.
However, in the past the provision of an elastic cushion has given rise to problems in anchoring the implant and in ensuring a uniform stressing of the peri-implant region.
It is known to provide an implant system using an elastic cushion and in which the implanted body adapted to be received in the jaw also receives, in turn, a so-called intramobile element in the form of an elastic cushion which is screwed into the implanted element or synthetic root.
The intramobile element essentially has the configuration of a cylindrical sleeve and is surrounded by a spacer sleeve which on the one hand is braced against the implanted element and on the other hand against the underside of a flange at the upper end of the intramobile element.
In a center screwthread of this cushion element, a so-called implantate post is screwed which constitutes the connecting member for the crown. The latter having a threaded pin, screw or bolt which can be screwed into an internal thread of the post. In this system, the intramobile element is stressed in compression by vertical stress in the flange region and in shear in the screwthread region. The result is a combined stress which is practically unpredictable or is poorly definable with mathematical-technical models.
The form of the intramobile element and the use of a homogeneous synthetic resin generally ensures that, at least with elastic deformation, this element undergoes deformation with a practically linear Hooke's law characteristic.
The threading of the intramobile element in the implant and of the crown connecting piece in the cushion is effected by hand so that fixed torques cannot be ensured.
Tests of these implant systems have shown that horizontal loading of the prosthesis, the horizontal yieldability is between 10 and 4 times less than the natural horizontal yieldability of molars. These tests have also shown that the elastic cushion which is about one hundred times too small by comparison with the internal mobility of molars.
The conclusion which is, therefore, inescapable is that the elasticity of the prosthesis with the intramobile element does not truly approximate that of natural teeth.